The history of quasar absorption lines began within a couple of years of the identification of the first quasar in 1963. In 1965, Gunn and Peterson considered the detection of flux blueward of the Ly emission line in the quasar 3C 9, observed by Schmidt, and derived a limit on the amount of neutral Hydrogen that could be present in intergalactic space. In that same year, Bahcall and Salpeter predicted that intervening material should produce observable discrete absorption features in quasar spectra. Such features were detected in 1967 in the quasar PKS 0237-23 by Greenstein and Schmidt, and in 1968 in PHL 938 by Burbidge, Lynds, and Stockton. By 1969 many intervening systems had been discovered, and Bahcall and Spitzer proposed that most with metals were produced by the halos of normal galaxies. As more data accumulated, the sheer number of Ly forest lines strongly supported the idea that galactic and intergalactic gas, and not only material intrinsic to the quasar, is the source of most quasar absorption lines.
In the 1980's many more quasar spectra were obtained and many large statistical surveys of the different classes of absorption line systems were published. The emphasis was to characterize the number of lines per unit redshift, dN/dz, stronger than some specified equivalent width limit. With 4m-class telescopes [equipped with charge coupled device (CCD) detectors] it was possible to conduct surveys with a spectral resolution of R ~ 1000. The spectral resolution is defined as R = / = c / v, so that R = 1000 corresponds to 300 km s-1 or 5 Å at = 5000 Å. Separate surveys were conducted for Ly lines, Mg II doublets, C IV doublets, and also for Lyman limit breaks, all as a function of redshift. The Ly line is observable in the optical part of the spectrum for z > 2.2, Mg II for 0.4 < z < 2.2, C IV for 1.7 < z < 5.0, and the Lyman limit break for z > 3. However, a break is also easily identified in lower resolution space-based UV spectra, which extended Lyman limit surveys to lower redshift.
In order to consider the cross section of the sky covered by the different populations, it can be assumed that absorption will be observed for all lines of sight within some radius of every luminous galaxy (> 0.05 L*K). (L*K represents the Schechter luminosity, i.e. the transition between the exponential and the power law forms of the luminosity function, and corresponds to a K-band absolute magnitude of MK = -25). To explain the observed dN/dz at z ~ 1.5, this radius would be 70 kpc for strong C IV (detection sensitivity 0.4 Å), and 40 kpc for strong Mg II (detection sensitivity 0.3 Å) and also for Lyman limit systems, implying that the latter two populations are in fact produced in the same gas. The higher N(HI) damped Ly absorbers would be produced within 15 kpc of the center of each galaxy, while the Ly forest lines would require a considerably larger region, hundreds of kpcs around each galaxy to produce a cross section consistent with the observed number of weak lines.
Up until the 1990's, the focus of quasar absorption line work was to separately consider the properties of the individual classes of absorbers (eg. Ly forest or Mg II absorbers). In the 1990's, however, three different observational advances led to recognition of the direct connections between the different classes of quasar absorption lines, and of direct associations with the population of galaxies:
1. Deep images of quasar fields could be obtained, and redshifts of the galaxies in the field could be determined from low resolution spectra. Steidel found that whenever Mg II absorption with Wr(Mg II) > 0.3 Å is observed, a luminous galaxy (LK > 0.06 L*K) is found within an impact parameter of 38 h-1 (L/L*K)-0.15 kpc with a redshift coincident with that determined from the absorption lines. Also, it is rare to find a galaxy within this impact parameter that does not produce Mg II absorption. There appears to be a one-to-one correspondence between strong Mg II absorption and luminous galaxies. The Mg II absorbing galaxies span a range of morphological types.
2. The High Resolution Spectrograph on the Keck I 10-meter telescope made it possible to obtain quasar spectra at a resolution of R = 45,000, which corresponds to ~ 6 km s-1. The previous surveys with resolution of order hundreds of km s-1 identified absorption due to entire galaxies and their environments. With 6 km s-1 resolution it became possible to resolve structure within a galaxy: the clouds in its halo, the interstellar medium of its disk, and the satellites and infalling gas clouds in its environment. Figure 4 is a dramatic illustration of this contrast for the Mg II absorber at z = 0.93 toward the quasar PG 1206+459.
Figure 4. Dramatic demonstration of gains due to high resolution spectroscopy of the Mg II doublet. The top panel is a R = 3000 spectrum of PG 1206+459. The doublet that is apparent at an observed wavelength of ~ 5400 Å is due to Mg II absorption from a system at z = 0.927. The middle panel shows the remarkable kinematic structure that is revealed at the resolution (R = 45,000) of the Keck/HIRES spectrograph of the same quasar. The 2796 Å transition is resolved into multiple components (5583-5592 Å), which also appear in the 2803 Å transition (5396-5406 Å). This system can be separated in two ``clusters'' of clouds, labeled ``A'' and ``B''. Another weaker Mg II doublet is observed at 5409 and 5423 Å, from a system at z = 0.934 Å, labeled with a ``C''. The solid line through these complex Mg II profiles is the result of multiple Voigt profile fitting, with a cloud centered on each of the ticks drawn above the spectrum. The lower panel shows the C IV doublets associated with the same three systems, observed with the Faint Object Spectrograph on HST, but at much lower resolution (R = 1300). The C IV is in three different concentrations around the three systems ``A'', ``B'', and ``C''. The C IV 1550 transition from system A is blended with the C IV 1548 transition from system B. The C IV equivalent width is too large for this absorption to be produced by the same phase of gas that produces the Mg II cloud absorption. The maximum absorption that can arise in the Mg II phase is given by the dotted line; a plausible model with a kinematically broader C IV phase yields the solid curve.
3. The Faint Object Spectrograph (FOS) on the Hubble Space Telescope provided resolution R ~ 1000 in the UV, from 1400-3300 Å. Observations of Ly forest clouds could be extended from z = 2.2 down to the present epoch. Furthermore, absorption from a given galaxy could be observed in numerous transitions; if Mg II was observed in the optical, the Lyman series and C IV could be studied in the UV (see Figure 4). With information on transitions with a range of ionization states, consideration of the degree of ionization (related to the gas density and the intensity and shape of the ionizing radiation field) and the multiple phase structure of galactic gas became possible.
No longer is analysis of absorption lines in quasar spectra an esoteric subject. It has developed into a powerful tool to be used in the study of galaxy evolution (eg. similar to imaging the stellar components of the galaxies). At least in principle, quasar spectra can be used for an unbiased study of the gaseous environments of galaxies from the present back to the highest redshifts at which quasars are observed. Gas structures smaller than 1 M can be detected if they are intercepted by the quasar line of sight, irrespective of whether they emit light. Through the tool of quasar absorption lines, proto-galactic structures and low surface brightness galaxies can be studied as well as high luminosity galaxies.